This article introduces the principle of cavitation in centrifugal pumps, the hazards of cavitation, and explains how to identify and prevent cavitation. 

Centrifugal water pumps are widely used in petrochemical, metallurgy, water conservancy, electric power, nuclear power and other industrial fields. They are used for power transmission of liquid media in various production devices, and their performance reliability plays a very important role in the normal operation of the device. Cavitation is an important phenomenon in the operation of centrifugal pumps. It is the most common problem affecting the reliability and service life of centrifugal pumps. It is also a huge obstacle that affects its development towards large flow and high speed. Therefore, cavitation has become the current An important topic in pump research.

               

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The principle of Cavitation Phenomenon

Cavitation is a kind of fluid dynamics phenomenon, the root cause is that the liquid has a partial pressure drop during the flow process, forming a low pressure zone. According to the knowledge of physics, for a certain liquid phase medium, it corresponds to a certain saturated vapor pressure Pv at a certain temperature, and vaporization occurs when the pressure of the medium is less than Pv. When the centrifugal water pump is running, after the medium enters the pump suction port, the pressure is gradually reduced before the impeller does not work on the medium. When the pressure drops to the saturated vapor pressure at the corresponding temperature, the medium will boil and vaporize, making the original flow A large number of bubbles appear in the medium, and the bubbles contain the vapor of the conveying medium and the air that was originally dissolved in the medium and escaped. When the bubbles flow from the low-pressure zone to the high-pressure zone with the liquid flow, the pressure of the medium rises rapidly due to the work of the rotating impeller on the medium. When the pressure is greater than the saturated vapor pressure Pv at the corresponding temperature, the bubbles will condense into a liquid phase again. A large number of cavities are formed instantly, and the surrounding liquid medium rushes towards the cavities at high speed and collides with each other, causing the local pressure at the cavities to increase sharply. This kind of liquid hammer is a kind of high-strength, high-frequency impact. Its pressure can reach hundreds of atmospheres and the frequency of water hammer is as high as 25,000 times per second. The material wall is subjected to such high-frequency and high-pressure repeated loads. Fatigue failure occurs gradually. Under certain working conditions, active gas (such as oxygen, etc.) may be dissolved in the pumped medium. When the medium is condensed from the gas phase to the liquid phase, a large amount of heat will be released, which will cause electrochemical corrosion of the metal and accelerate the rate of corrosion damage. , Resulting in pitting, perforation and even fracture on the metal surface. This phenomenon of vaporization, condensation, and impact of the liquid phase medium in the pump, resulting in the corrosion and damage of metal materials, is collectively called the cavitation of the centrifugal pump.

The harm of cavitation

Cavitation will affect the normal operation of the centrifugal water pump and cause many serious consequences.

First, Cavitation will reduce the performance of the centrifugal pump. The centrifugal pump transfers energy to the medium through the rotation of the impeller and converts it into pressure energy of the medium, but cavitation will cause serious interference to the energy transfer between the impeller and the liquid. When cavitation occurs, a large number of bubbles will be generated in the medium, which will block the flow path of the impeller, and generate vortices locally, which will increase the flow loss, which will reduce the flow rate, lift and efficiency of the pump. Flow, making the centrifugal pump unable to work normally. Judging from the performance curve of the centrifugal pump when the cavitation is severe in Figure 1, when the cavitation is severe, the performance indicators drop sharply.

Secondly, Cavitation will damage the wetted parts. Among the flow parts of the centrifugal water pump, the impeller is the most affected part by cavitation. When cavitation occurs, the surface of the metal material will gradually produce many small pits, and then the pits will continue to develop and expand into a honeycomb or groove shape. At times, perforations will be formed, and even the impeller will break, which will seriously affect the service life of the pump.

Thirdly, Cavitation causes pump noise and vibration. When cavitation occurs, high-frequency liquids collide with each other to produce various noises. In severe cases, crackling explosions will be emitted in the pump, and the vibration of the pump unit will be induced at the same time, and the vibration of the pump unit will accelerate the generation of bubbles and rupture. When the frequency of the liquid hammer is the same as the natural frequency of the pump unit, a strong cavitation resonance will occur and the amplitude will increase rapidly. At this time, if the centrifugal water pump is to be protected from greater damage, it must be shut down immediately for inspection.

Fourth, Cavitation restricts the development of centrifugal pumps. With the continuous advancement of science and technology, modern industries require centrifugal pumps to develop with large flow and high head. This requires increasing the flow rate of the medium. According to fluid mechanics, the higher the liquid flow rate, the greater the inlet pressure loss, and the more likely to produce cavitation. Therefore, improving pump anti-cavitation performance and studying cavitation mechanism are important research topics in the development of centrifugal pumps.

Preventive measures of pump cavitation

According to the conditions of cavitation, if you want to avoid cavitation in the centrifugal water pump, you should ensure that NPSHa>NPSHr, and a certain margin should be left. Accordingly, cavitation can be avoided by increasing NPSHa or reducing NPSHr during the design, manufacture and use of centrifugal pumps.

First, Improve the structural design of the pump. To improve the cavitation performance of the pump, you can start from reducing the necessary cavitation margin of the pump

Secondly, Improve the effective cavitation margin of the device. When designing the device, optimize the design as much as possible to increase the effective NPSH NPSHa at the suction port of the pump.

Thirdly, Use anti-cavitation materials or coat wetted parts. When the centrifugal water pump is restricted by working conditions and other factors and cannot completely avoid the occurrence of cavitation, materials with good cavitation resistance can be used to manufacture the impeller to extend the service life of the impeller. Practice has proved that the higher the strength and hardness of the material, the better the toughness, the more stable the chemical properties, and the better the cavitation resistance of the material. Commonly used materials such as stainless steel containing nickel and chromium, aluminum bronze, and high nickel-chromium alloys. In addition, it is much more economical to use epoxy-based anti-cavitation and wear-resistant materials to coat the surface of the flow-through parts of the centrifugal pump than to use expensive alloy steel.

Conclusion

Cavitation is an important factor that affects the normal operation and service life of centrifugal water pump. Understanding its principle and adopting appropriate measures to avoid the occurrence of cavitation can reduce or avoid the harm caused by cavitation. This article introduces the common measures to avoid cavitation. According to the specific process requirements and operating environment, appropriate measures should be adopted to improve the anti-cavitation performance of the pump. Hope it is useful for you!